1. Field of the Invention
The present invention relates to a semiconductor display device having a light-emitting element in each pixel.
2. Description of the Related Art
In an active matrix semiconductor display device, a switching element and a display element are provided in each of several hundred thousand to several million pixels arranged in matrix. Since application of voltage or supply of current to the display element is held to some extent by the switching element after a video signal is input to the pixel, the active matrix semiconductor display device can be flexible on enlargement and high definition and has become the mainstream of future semiconductor display devices.
As a problem caused in accordance with enlargement of a semiconductor display device, there is the potential drop of a wiring, which is caused by increase in a resistance value. For example, when the potential of a wiring (a scan line) connected to a gate electrode of a transistor which functions as a switching element drops, disorder is generated in the waveform of a signal input to the scan line, so that the switching of the transistor cannot be controlled at appropriate timing. In particular, gate electrodes of transistors included in all pixels in a horizontal direction are connected to the scan line. When the number of pixels is increased as a semiconductor display device has higher definition, the number of transistors connected to one scan line is also increased. Therefore, the potential of the scan line drops more significantly, which makes the appropriate control of the switching of the transistors more difficult.
If the scan line can be formed using a material having lower resistivity, the potential drop can be suppressed. However, a gate electrode of a transistor included in a pixel and a scan line are usually formed by processing (patterning) a conductive film formed over one layer into a desired shape by etching or the like. It is necessary that the gate electrode have heat resistance which is high enough to withstand heat treatment performed in steps of manufacturing the transistor. Thus, the kinds of materials which can be used for the gate electrode and the scan line have been limited to certain kinds.
Reference 1 (Japanese Published Patent Application No. H10-198292) discloses a liquid crystal display device in which the potential drop of a scan line is suppressed by connecting the scan line to an auxiliary wiring formed over a layer which is different from the layer over which the scan line is formed.